Talk:Neutron star

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Contents 1 Escape Velocity 2 Internal structure and questions 3 Neutronium and quark matter 4 Extraterrestrials 5 question 6 Rate of rotation slow down? 7 Effects of superstrong magnetic fields 8 Ionized electrons? 9 Size and mass? 10 Re: Journal Reference? 11 Why are some pulsars? 12 new intro... 13 Radius 14 Recent Changes 15 Neutron star structure 16 Clarification Edit 17 magnetic flux of neutron stars 18 update rotation speed 19 RRAT 20 Tsar Bomb 21 New discoveries about neutron stars 22 Cause of rotational speed 23 Magnetars and AmEx? 24 The hazards of falling on a neutron star 25 Ridiculous Analogy made of a human striking a neutron star, what sheer folly and nonsense 26 Vandalism

[edit] Escape Velocity

For the escape velocity of the neutron star, I wonder, was that found by using the traditional newtonian physics equation for gravitational potential energy, converting it into kinetic energy and finding the escape velocity? [ sqrt((2GM)/r) ] However, as it says, 150,000 km/s is half the speed of light, wouldn't you have to use general relativity to figure it out instead? Mercury has a much lower speed, yet even its orbit has certain relativistic effects. Are there any math geniuses that can confirm or refute the escape velocity of 150,000 km/s? It isn't important but it's sort of interesting to think about and I'd like to know. —The preceding unsigned comment was added by 72.59.2.127 (talk • contribs) .

I may be wrong about this, but under the escape velocity section it implies that an object falling into a neutron star would necesarily impact it at a velocity equal to the escape velocity. I don't see why this is true at all. The velocity of anything impacting the star would be dependent on a variety of factors, including other gravitational forces in the area, and distance from the neutron star when the object began "falling" towards it. As written it implies that if two objects were orbiting the star, one at 1 AU and one at 1.5 AU, and they both stopped their orbits and began falling, they would both impact at the same final velocity. This appears plainly false. If I am messing up my physics someone let me know, but otherwise, I'm going to change it. Not my leg 20:07, 8 June 2006 (UTC)

For all practical purposes, it's correct. While the impact velocity is only exactly equal to the escape velocity when the object is dropped from an arbitrarily far distance away, in practice, as long as the object is outside most of the potential well of the neutron star (in this case, more than a few hundred kilometres away), the difference in velocity is very small compared to the final velocity itself. --Christopher Thomas 20:56, 8 June 2006 (UTC)

[edit] Internal structure and questions

Question: what is the internal structure of a neutron star like? Where are the boundaries between neutronium, degenerate and normal matter located? What percentage of the stars mass or volume are each? What is degenerate matter? What do the boundaries between the layers look like? What is there shape? (I suppose some of these questions may not have answers yet...) -- SJK

The following are the transitions between the states you mention: it depends on the density: (in g/cm^3)

~1 x 10^6 - electrons become relativistic

~1 x 10^9 - nutronization

~4 x 10^12 - neutron degeneracy pressure dominates

~2 x 10^14 - nuclei dissolve

~4 x 10^14 - pion production: superfluid neutrons, superconducting free protons, relaticistic free electrons, other elementary particles (not well known, possible production of exotic matter) —This unsigned comment was added by 70.25.24.27 (talk • contribs) on 05:10, 23 March 2006.

I can answer some of these questions...

From what I've read, neutron stars don't have any internal structure. It's just neutrons, all the way through. The degenerate and normal matter layers are just a few meters thick on the surface. So it's something like 110% plus percent of the star's mass is neutrons. This is because nothing else can possibly exist inside there.

It's weird to think about them. A neutron star is essentially a titanic atom, which is held together by gravitational force instead of the weak nuclear force. -- The_ansible

that is incorrect - it is the strong nuclear force that binds nucleons together, not the weak nuclear force. —This unsigned comment was added by 70.25.24.27 (talk • contribs) on 01:43, 24 March 2006.

As I corrected in the article, a neutron star is not like an atomic nucleus, because it is not just protons and neutrons, but almost all neutrons inside and whatever on the surface (assuming you're correct in that notion).

Also I think you meant the residual strong force, the weak force is the mediator of beta decay, the breaking up of neutrons into electrons etc., the opposite of what happens in a neutron star. Rmrfstar 00:36, 5 Apr 2005 (UTC)

Actually, I would disagree. The outermost layers of a neutron star will consist of neutrons in a sort of crust, but as you look further inside, the density and pressure get much higher, and the neutron structure dissolves to leave a sort of deconfined quark core. Around this phase transition density, it is energetically favourable to 'create' hyperons and other baryons. There is a lot more to a neutron star than just neutrons. A heck of a lot less than 99.9%. Besides, for interactions between the neutrons you will need sigma, omega and rho mesons.

The comment "nothing else can possibly exist inside there" is most certainly incorrect as far as the most recent theories go. As for the exact structure of a neutron star, that is purely left up to theory, though limits can be imposed based on observations. If you could crack a neutron star open to have a look, please let me know! -- JDC

if the core of a "neutron star" is not made up of neutrons (excluding bosons needed for interactions between the neutrons) then it should not be called a "neutron star" "" 16:55, 21 May 2006 (UTC)

Like it or not, it's going to keep being called a "neutron star", just like Pluto is going to keep being called a planet no matter how much other junk we find in the Kuiper belt. Furthermore, a large fraction of it _does_ consist of neutron-degenerate matter, exotic stuff in the core notwithstanding, so the name does still remain appropriate.

Regarding contents of the core, my understanding is that this depends quite strongly on the equations of state of neutron-degenerate matter, quark matter, and the other forms of exotic matter that might be present, so it's far from certain what's actually in there (could be mostly neutrons, could (less likely) be mostly quark soup or strange matter; pin down QCD parameters and do more simulations to find out). So, any statement of "almost all X" should be taken with a grain of salt pending a better understanding of the phases of matter involved. --Christopher Thomas 06:11, 22 May 2006 (UTC)

[edit] Neutronium and quark matter

Whoever deleted the term 'neutronium' from the main page -- if you are saying not to use that term, note we already have a page neutronium. Secondly, I find your new version hard to follow -- whats an 'iron layer'? Finally, but 'quark matter' you mean what? -- SJK

more on neutronium - http://www.physics.uq.edu.au/people/ross/ph227/evolve/whitey.htm

"quark matter" is probably a reference to strange matter

Sorry, it was made in a haste. Iron layer is a thin crust of metalic iron, by quark matter i mean a soup where not even individual neutrons exist, but a mixture of quarks of different kinds. "strange" matter is matter partially composed by "strange" quarks. What i tried to emphasize is that there was a comtinuum of compositions inside a neutron star, and that telling "a crust of degenerate matter and and interior of neutronium" was an oversimplification that also hided the fact that there is no aggreement respect to what's in the core. AN

Ok. i can live with that for the time being. AN

You see - we actually can all get along :) - MMGB

[edit] Extraterrestrials

From the article:

"When neutron stars were first discovered, they were believed to be evidence of extra-terrestrial intelligences. Because of their highly regular pattern of emmisions, they were initially though to be beacons of some type."

I don't think that pulsars were ever seriously thought to be evidence of extraterrestrial intelligence, at least not by the researchers who were actually investigating them. The first pulsar discovered was dubbed "BEM-1" (Bug-Eyed Monster 1) as a joke by its discoverers, if I recall correctly. But since this is all from memory, anyone have any references handy? Bryan Derksen

Actually it was called LGM-1 (little green men). According to the article pulsar they did seriously consider it. The reference used is astrophysicist Perer Sturrock but with his interest in UFOs I don't know how reliable the reference is.

An interesting article can be found here [1] which I was directed to from a NASA site. It is a description of the discovery by Jocelyn Bell Burnell herself. Of note is the passage that reads

"In the paper to NATURE we mentioned that at one stage we had thought the signals might be from another civilization. When the paper was published the press descended, and when they discovered a woman was involved they descended even faster. I had my photograph taken standing on a bank, sitting on a bank, standing on a bank examining bogus records, sitting on a bank examining bogus records: one of them even had me running down the bank waving my arms in the air - Look happy dear, you've just made a Discovery! (Archimedes doesn't know what he missed!) Meanwhile the journalists were asking relevant questions like was I taller than or not quite as tall as Princess Margaret (we have quaint units of measurement in Britain) and how many boyfriends did I have at a time?" Unmasked 01:55, 30 October 2006 (UTC)

[edit] question

As a non-physicist, how does this sentence:

Neutron stars are the densest objects known

relate to black holes? Thanks, [[User:Meelar|Meelar (talk)]] 19:04, Sep 10, 2004 (UTC)

I'm not well read in astrophysics, but mass and [density] are two separate things. Compare traffic in New York to traffic in California. New York has tons of traffic jams and everyone is stuck together. California might have more cars overall, but they're more spread out for the main part. The same is true for black holes and neutron stars, black holes have more mass, but neutron stars are more closely packed. Presuming the statement above is correct. If anyone more knowledgable can confirm this that would be good.

User:zdude255

I believe you are correct in your distinctions between mass and density, however I think a black hole is still infinitly dense, even if you determine it's size by it's Schwartzchild radius. Rmrfstar 00:38, 5 Apr 2005 (UTC)

It's easy to show that it's not infinitely dense if you're using the size of the event horizon, simply by observing that both the mass and the radius are finite and nonzero. Divide the mass by the volume to get density. While black holes the mass of a star are extremely dense, the fact that black hole radius is directly proportional to mass means that density drops off as the inverse square of mass. A supermassive black hole containing a billion solar masses would be only a little denser than air. In practice, calculating density this way doesn't tell you anything terribly useful, but it's interesting to think about. --Christopher Thomas 08:50, 2 March 2006 (UTC)

The real problem, though, with black holes, is that we have never seen a black hole; no one has, because it is impossible to observe one, and we can only see their effects. So, in a sense, black holes are not "known", per se, while neutron stars can be and have been observed directly, even if what we know is, in fact, based on mostly mathematical models. —The preceding unsigned comment was added by 67.160.131.117 (talk • contribs) on 08:04, 2 March 2006.

We haven't observed neutron stars directly, any more than we've observed black holes directly. In both cases, we see that there's a massive, compact object affecting nearby matter. Mostly what we see are the accretion disc, a black hole's polar jets, radio and microwave emissions from particles caught in a neutron star's magnetic field, and glowing from infalling matter impacting the surface of a neutron star (which is absent for black holes, as they have no surface to impact on). From the motions of any companion stars, the mass of the neutron star or black hole can be estimated. From the temperature and velocity of various parts of the accretion disc, its radius can be estimated (though often it's just assumed to be in the expected range for the given mass, as it's difficult to get a reliable direct measurement).

For that matter, nobody's directly observed an "electron"; only measured its effects. Yet we're still reasonably confident that electrons exist. --Christopher Thomas 08:50, 2 March 2006 (UTC)

Indeed, one must define "observe" carefully when speaking of physics. Very little in physics is directly observable by the human eye, but much is quite certain nonetheless. Most of steller physics is known through taking spectra of stars for example - we don't "see" what stars are made of, but the science tells us clearly by examing absoption/emission lines of stars and comparing to the quantum nature of various elements, also deduced through mathematics and observations of careful experimentation.

First off, density ~ Mass/Radius^3. The above discussion is confusing the physics: Stars begin their life as "fluffy" balls of gas, slowly reacting to create slightly heavier elements than the Hydrogen and Helium they begin with. This process of nuclear fusion keeps powering the star over its Main Sequence, or primary lifespan. Depending on the star's mass, it will have a proportional amount of energy during the inevitable and eventual collapse, once nuclear fusion runs out. This occurs once the mass avaliable is insufficient to continue to raise the temperature enough to promote more fusion, or if the fusion process has gone all the way to Iron - the most bound of all atomic nuclei. There can be no fusion after reaching Iron, so the pressure keeping the star from collapsing under gravity disappears, and the star collapses. When it collapses the density increases dramatically. Due to Heisenburg's Uncertainty Principle, electrons and neutrons can only be "pushed together" so much. If the density reaches a critical point, electron degeneracy pressure maintains the stars integrity, and the star becomes a white dwarf. It the white dwarf has more than 1.4 solar masses (whether at time of initial collapse or from accretion), it was collapse because the gravitational force will overcome the electron degeneracy. At this point, neutron degeneracy can take over and support the star, creating a neutron star. If the neutron star accretes mass somehow, it would collapse eventually into a blackhole. —This unsigned comment was added by 70.25.24.27 (talk • contribs) on 05:04, 23 March 2006.

[edit] Rate of rotation slow down?

The article specifies slow down rates that appear far, far too small (10-12 and 10-19 second for each century), in fact too small to logically produce the older slower-rotating stars within the universe's current age the article also mentions. Googling around, I see zebu.uoregon.edu has "about 10-15 seconds per rotation", so I suspect "per century" should be changed to "per rotation", and this would change the next sentence as follows, for a star initially rotating at 1 second: In other words, a neutron star now rotating in 1 second will rotate in 1.000003 seconds after a century, or 1.03 seconds after 1 million years. Does this look sensible? -Wikibob | Talk 22:49, 2005 Apr 19 (UTC)

This is kind of true, however generally the spin-down rate (period derivative) is expressed in dimensionless units, or if you want to think of it that way, seconds per second (=rotations per rotation). What you've said is kind of true since most observable neutron stars have rotation periods of the order of 1 second. However, on that point, both the spin period AND the period derivative are very much dependent on the type of neutron star (i.e. its history and present circumstances). So probably the whole paragraph needs to be generalised a bit, and the specific numbers and info left to the appropriate sub-articles. Rotation-powered pulsars spin down via magnetic dipole radiation, so their spin down rate depends on their magnetic field strength. The range you quoted is roughly valid for ordinary rotation powered pulsars, which have periods of mainly about 0.1-5 seconds. Recycled rotation powered pulsars otoh have periods of 1-100 milliseconds and spin down rates of 10^{-21 - -17}, while magnetars have periods around 10 seconds spin down of 10^{-12 - -10}. Accreting neutron stars OTOH vary in spin period due to the transfer of angular momentum from the accretion stream, so this can be spinning up or down and can vary with time, and have spin periods up to many 10s of seconds. All this is IMO too much detail for the top level neutron star entry (but I don't have time at the moment to improve it myself). Rkundalini 07:07, 21 Apr 2005 (UTC)

[edit] Effects of superstrong magnetic fields

The Neutron star article states:

"Another class of neutron star, known as the magnetar, exists. These have a magnetic field of above 10 gigateslas, strong enough to wipe a credit card from the distance of the Sun and strong enough to be fatal from the distance of the Moon. By comparison, Earth's natural magnetic field is 50 microteslas, and on Earth a fatal magnetic field is only a theoretical possibility; some of the strongest fields generated are actually used in medical imaging. A small neodymium based rare earth magnet has a field of about a tesla, and most media used for data storage can be erased with milliteslas."

The Magnetar article states:

"A magnetic field above 10 gigateslas is strong enough to wipe a credit card from half the distance of the Moon from the Earth1. A small neodymium based rare earth magnet has a field of about a tesla, Earth has a geomagnetic field of 30-60 microteslas, and most media used for data storage can be erased with a millitesla field.

The magnetic field of a magnetar would be lethal at a distance of up to 1000 km, by warping the atoms in living flesh2."

The two don't agree too well, though they are clearly derived from similar source material. Maybe someone who knows which version is correct can fix the incorrect one? Thanks!--Ailicec 01:18, 7 Jun 2005 (UTC)

Wouldn't the iron in your blood be affected, Xmen style, before the "atoms in living flesh" were warped? If that statment is untrue it would detract from the credibility of the 1000km statement. Rmrfstar 21:30, 19 May 2005 (UTC)

The main point being that at least one of them is wrong. ((btw, added my sig to original question, I didn't know how to do it when I wrote that) --Ailicec 01:18, 7 Jun 2005 (UTC)

[edit] Ionized electrons?

"The matter at the surface of a neutron star is composed of ordinary nuclei as well as ionized electrons." Surely the person writing this meant "ionized atoms" eg ions?

[edit] Size and mass?

Early in the article: "Neutron stars have a mass of the same order as the mass of the Sun. Their size (radius) is of order 10 km, about 70,000 times smaller than the Sun." Later in the redundant 5th paragraph: "Neutron stars are typically about 20 km in diameter, have greater than 1.4 times the mass of our Sun"

This site:1 says it's 10km

But this one:2 says it must be at least 18km

Merick June 29, 2005 15:52 (UTC)

[edit] Re: Journal Reference?

In 1933 Walter Baade and Fritz Zwicky (Phys. Rev. 45 "Supernovae and Cosmic rays") proposed the existence of the neutron star, only a year after Chadwick's discovery of the neutron. In seeking an explanation for the origin of a supernova, they proposed that the neutron star is formed in a supernova.

Should this reference be

'Remarks on Super-Novae and Cosmic Rays'
W. Baade and F. Zwicky
Phys. Rev. 46, 76-77 (1934)

The paper was in Phys.Rev. in 1934, but the discovery was still in 1933.

Keep up the good work!!!

-- JDC

[edit] Why are some pulsars?

Is it known why some neutron stars are pulsars and some aren't? Or are they all, and it just isn't observable?

Response:

If I may, there are several models of static neutron stars (non rotating) which are still used to date to model parameters, which would not be considered 'pulsars', since any received beam of radio waves wouldn't 'pulse'. I suppose the question is - will a neutron star emit the radio waves if it isn't rotating?

One of the main advantages of pulsars is that they can be detected via these pulses. Neutron stars are otherwise very small, very dark, and extremely hard to find.

-- JDC

[edit] new intro...

Concerning Fxer's new introduction, I'm ambivilant about keeping it. The new intro is less of a definition and is slower to give a clear idea of what a neutron star is and its importance (first theororized astromical object). For instance, that new sentence of the "weight" of a spoonful of a neutron star: Is that the kind of statistic we want in the intro paragraph? Also there is now presented irrelevant information on black holes and white dwarfs, which should be mentioned later... On a more positive note, the newer introduction correctly names Neutron stars as types of degenerate stars. Any other thoughts? -- Rmrfstar 00:17, 21 July 2005 (UTC)

I'd be all for a rewritten version of the intro :) It was rewritten partially, and only confused things more, so I changed it a bit to at least be factual. The statistic about the "weight" was just an eye catcher, put the most interesting info forward, and stuff people can relate to. Changing the sentence about black holes to something like "one outcome of a supernova is a neutron star", or "the supernova of a medium sized star" could be preferrable, and move the other possible outcomes to later in the body. I only started editing this page because it had no image...I hate articles without pictures, humans are visual creatures ;) --Fxer 00:37, July 21, 2005 (UTC)

Part of the current intro is confusing to the lay reader (namely, me):

A typical neutron star has a mass between 1.35 to about 2.1 solar masses, with a corresponding radius between 20 and 10 km (they shrink as their mass increases) — 30,000 to 70,000 times smaller than the Sun. Thus, neutron stars have densities of 8×10¹³ to 2×10¹⁵ g/cm³, about the density of an atomic nucleus.^[1] Compact stars of less than 1.44 solar masses, the Chandrasekhar limit, are white dwarfs; above three to five solar masses (the Tolman-Oppenheimer-Volkoff limit), gravitational collapse occurs, inevitably producing a black hole.

Now, I'm hardly an astrophysicist, but I'm having trouble understanding exactly how the Chandrasekhar Limit applies to neutron stars specifically. It doesn't make sense that a neutron star with a mass below the limit would utlimately collapse into a white dwarf. How exactly does the Limit apply - can someone clarify? UndercoverParrothead 22:48, 14 October 2006 (UTC)

[edit] Radius

Would anyone object to the radius of a neutron star being pushed down to 10km? This is the research I am doing at the moment, and literature / my results point to 10km. Cheers.

-- JDC

[edit] Recent Changes

To the person/people who continually and intentionally add errors to WikiPedia pages. Please stop. There is an entire encyclopedia out there for you to destroy. Please go to uncyclopedia or kamelopedia if you are going to ruin pages. These are here for genuine interest.

--JDC

[edit] Neutron star structure

Will some knowledgeable person please either add some references establishing the "atmosphere/crust" structural version of a neutron star, or else rewrite it to remove that wording? I'm no astrophysicist, but as a physics grad student and a follower of such things as much as I have been able in the scientific and popular literature, I have never heard of such a thing. Not saying it's false, just that we ought to have some sort of documentation.

Would a reference to the research on neutron star crusts suffice? Although almost every aspect of neutron stars is theory, we just try to make the theory better and better. The crust appears to make a dramatic difference at low central densities, where the neutron matter is below the neutron drip density, and finite nuclei are still stable. The common reference is

The Ground State of Matter at High Densities: Equation of State and Stellar Models

G. Baym, C. Pethick, P. Sutherland

Astrophysical Journal, vol. 170, p.299

since this is widely used to calculate the equation of state for the crust. You probably won't find much about crusts in the literature though. Should this go in, or is this too involved?

--JDC 00:32, 4 October 2005 (UTC)

[edit] Clarification Edit

The edit involving the 'fudge factor' is fine by me. The only observational parameters we have (to a decent degree) are mass limits. We construct models to one day test via experiments.

--JDC 23:52, 4 October 2005 (UTC)

[edit] magnetic flux of neutron stars

Can someone please tell me how neutron stars get their huge magnetic flux. I don't see how something made up of mostly neutrons can have a magnetic flux so large with an area of so small. Where exactly does the magnetic flux come from? I can't seem to find any articles or publications on this matter.

Neutron stars have high magnetic flux density. A lot of positive and negative charges in a small volume means a really dense object which implies a really dense electromagnetic field.Kmarinas86 19:45, 25 December 2005 (UTC)

Any energy is controvertible to any other form of energy. Field cancellation often leads out a detectable residual force. Color charge leads to a residual strong force which may lead to a residual electromagnetic force which leads to a residual Van der Waals force which leads the early formation of solar systems and more, and so on. I wonder what residual forces have to do with the containment of energy. —The preceding unsigned comment was added by Kmarinas86 (talk • contribs) .

My understanding is that this is best understood in terms of the same effect that allows you to build an electromagnetic pulse weapon based on explosively pumped flux compression generators: When you compact the star's core, its magnetic field doesn't go away. Instead, you're trying to stuff as much energy as was bound in the original field into the magnetic field of an object far, far smaller. As a neutron star does have free charge carriers (an electron gas on the surface and a relatively small number of protons and electrons in the interior), there isn't any physical reason preventing the star from having a magnetic field. --Christopher Thomas 21:21, 25 December 2005 (UTC)

[edit] update rotation speed

New Scientist and Physics Web report the discovery of a neutron star that rotates 716 times per second. (newscientist.com 12 january 2006)

The current "one revolution can take anything from thirty seconds to one six-hundredth of a second" could be updated to reflect this (seven-hundreth of a second) —The preceding unsigned comment was added by 212.123.21.4 (talk • contribs) on 19:34, 6 February 2006.

[edit] RRAT

New stars found called Rotating Radio Transients, or RRATs http://news.yahoo.com/s/space/20060215/sc_space/astronomersdiscoverpeekaboostars Can someone create a new article? —The preceding unsigned comment was added by 66.25.142.153 (talk • contribs) .

It would probably end up being a redirect to this article, with a section added to the neutron star page for rotating radio transients of the type described in the article. If you can provide citations to journal papers or university-hosted web sites about these stars, though, I'm sure it'll get added pretty quickly. Welcome to Wikipedia, and happy editing! --Christopher Thomas 03:09, 16 February 2006 (UTC)

[edit] Tsar Bomb

I have edited the section that says the Tsar bomb was 100 MT. The bomb (in the form tested) was only 50MT. There was a plan to make a 100MT 'dirty bomb' but this was never tested - nor created I think.

The difference between those two versions was an uranium tamper which was exchanged shortly before detonation for a lead tamper. So with all respect one can safely say that it is the biggest bomb that was 'built' by mankind, though fortunately not detonated. Knowledge about those bombs was advanced enough at the time to support the untested 100 Mt figure. Endymi0n 18:58, 20 March 2006 (UTC)

[edit] New discoveries about neutron stars

I think we could take this into consideration: http://www.space.com/scienceastronomy/060501_mm_starquake.html It's probably nothing significant, but I just wanted everyone to know. —The preceding unsigned comment was added by Uni4dfx (talk • contribs) on 12:00, 1 May 2006.

[edit] Cause of rotational speed

"Due to its small size and high density, a neutron star possesses a very high rotation speed..." Isn't the high rotational speed due to conservation of angular momentum in conjunction with the drastically reduced radius of the star? Clarityfiend 08:39, 9 June 2006 (UTC)

Yes, and that's what "small size and high density" refer to, so I'm puzzled by your question. If you think that could be phrased more clearly, go ahead and edit it. --Christopher Thomas 16:25, 9 June 2006 (UTC)

One doesn't imply the other. My thanks to Xerxes314 for fixing it (more accurately than I would have). Clarityfiend 02:51, 11 June 2006 (UTC)

[edit] Magnetars and AmEx?

The credit card analogy in the magnetars section is unclear. Does it mean 'wipe' as in demagnetise? Or is 'wipe' a typo for 'swipe', as in it could read the CC info from that distance?--Anchoress 01:10, 24 June 2006 (UTC)

It means demagnetize. Numbers for this are suspect, but are still useful for making the strength of the field clear to readers who aren't experts. --Christopher Thomas 06:16, 24 June 2006 (UTC)

[edit] The hazards of falling on a neutron star

To the sentence about what it would be like to fall on a neutron star:

if an average human were to encounter a neutron star, he or she would impact with roughly the energy yield of a 100 megaton nuclear explosion (a power equivalent to twice the tsar bomba, the biggest nuclear weapon ever detonated)

I had added the comment: and probably not survive.

This contribution of mine to Wikipedia lasted only about 15 minutes; it was reverted by Bryan Derksen, saying:

a human _probably_ wouldn't survive falling onto a neutron star's surface? Only taxes are as certain. :))

This seems to imply that there is no place for humor on WP, no matter how obvious it might be (my comment certainly could not be taken for serious and mislead someone - let alone lead to serious consequences, like someone really going off to jump on a neutron star, more on that below). Surely someone is going to point to some Be dead serious with a straight face official guideline about that that I overlooked.

However, just for revenge, I want to say that Bryan's own comment is itself misleading; indeed, it is false:

• Taxes are not certain. I hope I won't loose my job, but if I do I won't pay taxes any more. I might also just die (even without the help of a neutron star).

• It is simply not true that a human being would be killed by the impact with a neutron star. That is just simply not true. The human would already be dead before impact. You can't be killed if you are already dead. The human would have been torn apart during descent by the tidal push and pull of the star's gravity.

• The human couldn't get there anyway. The closest neutron star must be light years away, and to get there, even with a modern high-speed train, would take at least tens of thousands of years. Even if you are young and start out today, you very probably will not survive that long. So again, you will not be killed by the impact.

It might be objected that if it is indeed impossible for a human to fall on a neutron star, then any sentence starting by "if an average human were to encounter a neutron star" is true. All implications are true from a false premise. But then it would be just as true to say the impact would produce the energy of a 1000 gigaton bomb, or that of lighting a match. That in turn would mean that the whole sentence is irrelevant, devoid of content. That cannot be the case, see the Everything on Wikipedia is 100% reliable relevant and perfect guideline. So this objection is dismissed.

Now, can I put my comment back?

David Olivier 13:11, 25 June 2006 (UTC)

No, I think it's still inappropriate. While humor is not exactly forbidden on Wikipedia, lack of clarity and deliberately misleading statements are. Your statement that a human would "probably not survive" impact on a neutron star suggests that there is some nonzero chance that a human could survive impact on a neutron star. That's simply not true. Your other points above aren't particularly relevant since this is a hypothetical scenario; it doesn't matter how the human got there, it's simply a given that he did. And it doesn't even matter whether he's alive when he hits, the result is the same either way. Bryan 06:32, 26 June 2006 (UTC)

I agree that it shouldn't be included. IMO it's not because there's no place for humour in WP, nor is it because of the inaccuracy of the statement. It's because it's irrelevant. The bomb analogy is appropriate because it conveys information about the topic, your addition does not.--Anchoress 04:50, 27 June 2006 (UTC)

[edit] Ridiculous Analogy made of a human striking a neutron star, what sheer folly and nonsense

I'd like to get rid of this metaphorical and visual reference of a human striking at 100megatons a neutron star, i mean if its mathematically and theoretically correct, in terms of the velocity, and energy, then one would require workings to show such an impossible situations, in the future of our race, as a moral relativist, this part should be deleted or expanded!!!The Idiot 14:21, 26 July 2006 (UTC)

I have deleted that sentence, not so much by opposition to moral relativism (I'm not sure I got your point), but because it is clearly misleading. It evokes a picture of a gigantic explosion (four giant hydrogen bombs), whereas in fact the matter falling on the surface would probably just flop into it, producing a small ripple (less than 1mm high), and perhaps ejecting a "splash" of particles travelling at relativistic speeds, which means it would be gone in an instant. A rather unspectacular event! That is because given the enormous density of the matter of the star, and the enormous gravity, it takes an enormous amount of energy to produce even minuscule perturbations.

I think it is quite enough to say that matter falling on the star will arrive at the surface at some 150,000km/s. (I also replaced "object" by "matter", because I don't think it has much sense to speak of an object in that context - the energy that keeps an ordinary object together is infinitesimal in regard to the energies in play.

David Olivier 21:32, 14 October 2006 (UTC)

I can't see how it's misleading, and I object to the removal of "real world" examples like this - they are useful for helping laypeople understand the magnitudes of the forces involved. The energy content of matter travelling at 150,000 kilometers per second could be hard to grasp otherwise. If you can find a ref or supporting calculations for the resulting release of energy raising material only a millimeter, or the flash being "gone in an instant" (I'm a little dubious of this since all that energy needs to be radiated by the rather small surface area of the neutron star), then we should definitely include that too because it'll provide even more good illustration of the strength of gravity involved. I'm restoring it with a more vaguely-worded caveat in the meantime. Bryan 22:23, 14 October 2006 (UTC)

OK, calculate. Taking the information already in the article, the density of the neutron star matter is at least 8×10¹³g/cm³, so one cubic millimeter has a mass of at least 8×10⁷ kilograms. The surface gravity is at least 2×10¹¹ that of the earth, i.e. at least 2×10¹² J/kg/m. That means that to raise one cubic millimeter of matter by one millimeter, you need at least 16x10¹⁶ joules. That is about one fifth of the energy stated for 70kg of matter falling at half the speed of light. In other words, that "200 megaton explosion" would be enough to just statically lift one cubic millimeter of matter half a centimeter high, in the most favourable case.

There are several ways for the energy of falling matter to be dissipated: surface ripples, compression waves radiating downwards through the star, local heating, a splash of ejected matter... In any case, it seems clear enough that the ripples produced will be lower than one millimeter, and that the whole event will be very different from what a layperson will imagine when you speak of a 200 megaton explosion. No billows of smoke, no giant craters, no charred neutronium.

As for the issue of the flash being gone in an instant: that is fairly obvious. The speeds involved are of the order of c, and any matter ejected, even at a speed as low as one hundredth of c, will in effect be gone in an instant, from the point of view of the (highly improbable) layperson sitting on a chair on the star and observing the event: in one tenth of a second it will already be hundreds of kilometers away. Actually, it will probably have fallen back on the star itself. In any case, the scene, again, will look nothing like what the example suggests.

As you say, examples can be useful helping a layperson understand the magnitude of the forces. That is a good thing, if the examples are not themselves misleading. I think the current example of a person falling on the star is deeply misleading. It gives the impression of some magic enormity in the energy you get falling on the star; it gives a very false image of what that really means, because it gives no real impression of how things really are on the star.

Actually, I think that any layperson can understand that 150 thousand kilometers per second is very fast, and that encountering any object at that speed is dramatic. I don't think that the picture of a nuclear explosion helps much at all the layperson to visualize what that amount of energy means (for a layperson, the term energy doesn't have a very clear meaning, and the idea of a nuclear explosion doesn't make it any clearer - it just adds notions of smoke and destruction, which are not relevant in this context).

It would be more interesting, I think, to elaborate a bit on the enormous density of the matter, and on the enormous gravity. One tiny bit of that matter (a cube one tenth of a millimeter wide) has a mass of at least 80 tonnes, and a weight at least 200 billion greater than what that mass would weigh on earth. Saying that is helpful, I believe, for the layperson who wishes to have an idea of what a neutron star is.

David Olivier 11:42, 15 October 2006 (UTC)

On the other hand, the 200 megaton explosion will lift 150 kilograms of matter almost three kilometers, which on something the size of a neutron star could give rise to a noticeable plume. The numbers I used: ((200 megatons * (4.184 * 10^15) J/ megaton ) / (2*10^12) Kg/J/m) / 150 kg = 2789 m. It's not clear to me what would determine how much of the infalling matter and how much of the crust matter would go into the initial explosion. As for the "instantaneousness" of the explosion, I believe you're contradicting yourself; either the ejected material is moving on the order of c and can escape the neutron star or it can only raise a short distance in which case it's confined to a thin "atmosphere" around the star. And if it's confined by the various mechanisms you describe, it may still be noticeable; a neutron star only has a surface area of 42 to 84 square kilometers, so if it's got 200 megatons of energy to radiate it has to dump somewhere around 10^9 J per square meter. The article doesn't indicate what sort of temperature a neutron star's surface is at usually, an omission that could use filling.

Anyway, perhaps we should take this stuff out of the intro paragraph and put it somewhere in the article where all these caveats and additional details could be added without worrying about using too much verbiage. If you're concerned about the layperson getting the wrong idea about the effects of such an explosion on a neutron star, IMO the proper approach is to try explaining what the effects would actually be rather than removing all mention of it and hoping the reader never thinks about such things. The energy content of infalling matter due to gravity is a significant fact about neutron stars, it comes up in other situations. Bryan 19:31, 15 October 2006 (UTC)

I also think that the analogy is tortured and confusing. More than anything I think it's not a "real world" example of what would happen if a human struck a neutron star and it's misleading to suggest that this is what would happen. No, this is just what you get when you plug some uncommonly big numbers into a basic physics formula, and then divide by the yield of a big nuclear bomb. It doesn't help anybody understand what's really going on -- if anything it confuses the issue by suggesting that a human body could get close to a neutron star and that we could measure the effect of the impact. Accordingly, I am removing it. Eliot 20:13, 24 October 2006 (UTC)

[edit] Vandalism

There's now been 3 instances of vandalism by 172.142.102.49. This is the only page he's editing, but he keeps editing the pulsars section to refer to penises. Took me a second to work it out. Lawful Hippo 04:55, 27 November 2006 (UTC)